Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles are typically equipped with lean exhaust gas aftertreatment devices, such as, for example, Active Lean NOx (ALNC) Catalysts, which are capable of continuously reducing NOx emissions in an oxygen rich environment. In order to maximize NOx reduction in the ALNC, a hydrocarbon-based reductant, such as fuel, is introduced into the exhaust gas entering the device via a reductant injection system. Typically, the amount of reductant injection is based on operating conditions, such as engine speed, load, ALNC temperature, etc. It is desirable to precisely control reductant injection amounts since underinjection of reductant may cause reduced NOx conversion efficiency of the ALNC, while overinjection results in unnecessary fuel economy penalty.
The inventors herein have recognized that degradation of the reductant injection system (such as leaking or clogging injectors, for example) and degradation of the ALNC due to, for example, aging or thermal damage, may cause over- or under-injection of reductant.
In that regard, the inventors have recognized that once the ALNC has reached certain operating temperatures, injection of reductant will create a temperature rise, or exotherm, across the ALNC due to hydrocarbon combustion in the device. The inventors have further recognized that it is possible to both diagnose degradation in the emission control system-and to identify the component responsible for the degradation by monitoring the rates of change of the exotherm across the ALNC.
Accordingly, the present invention is directed to a method for diagnosing degradation an emission control system for a lean burn internal combustion engine, the system including an Active Lean NOx Catalyst (ALNC) and a reductant injection system coupled upstream of the ALNC, the method including: calculating a rate of change of an exotherm across the ALNC; and differentiating between the ALNC and the reductant injection system as a cause of the emission control system degradation based on said calculated rate of change of said exotherm.
In one embodiment of the present invention, the method includes providing an indication that the reductant injection system is leaking if the rate of change of the exotherm is greater than a first positive value. In another embodiment of the present invention, the method includes adjusting the amount of reductant injection into the device to compensate for the leaking. In yet another embodiment of the present invention, the method includes adjusting the injected reductant amount to compensate for reduced efficiency if the rate of change of the exotherm is less than a first negative value and greater than a second negative value. In another embodiment of the present invention, the method includes disabling the reductant supply to the reductant injection system if the rate of change of the exotherm is less than a second negative value.
The present invention provides a number of advantages. In particular, monitoring the rate of change of the exotherm provides an accurate indication of system degradation that is not susceptible to transient exotherm changes due to instantaneous variations in the amounts of injected reductant. Yet another advantage of the present invention is the ability to identify the component responsible for the emission system degradation and to take appropriate corrective measures. Further, accurate and fast detection of the emission system degradation improves fuel economy and emission control in the vehicle.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.